This invention relates to direct current to alternating current, pulse width, modulated power converters.
Power conversion problems are encountered when it is necessary to utilize equipment designed for alternating current power in an environment where only direct current power is available or where a direct current primary power link is purposely employed to provide a battery backup in the event alternating current power becomes unavailable. In these applications a direct current to alternating current power converter is needed to interface the equipment to the primary power system. Virtually all modern alternating current equipment operate from either 115 or 230 volts AC.+-.10% at either 50, 60, or 400 Hz.+-.5%. In general, direct current primary power supplies are normally within the 11 to 32 volts DC range. This includes, for example, most modern aircraft and vehicular equipment.
In the prior art, the simplest approach to power conversion is to use a 60/400 Hz inverter followed by a low frequency transformer and filter. Regulation is achieved by preceding the inverter with an ultrasonic switching regulator or alternatively achieved in the inverter itself by varying the duty cycle of the inverter. In the case of the latter approach, the output filter must accommodate the worst case harmonic content of the signal's waveform which usually occurs at maximum input voltage. In the case of the former approach, the burden on the output filter is reduced because the DC level to the inverter is held constant. However, the switching regulator adds to the cost and degrades the efficiency of the overall power converter. In either case, the output filter must provide significant attenuation at the fundamental output frequency in order to provide a sinusoidal waveform.
The next level of sophistication of power converter design, in the prior art, is not only to vary the duty cycle of the inverter, but to chop the inverter's output signal into pulses with variable pulse widths by means of ultrasonic switching by using either feedback control against a sinusoidal reference or by "opening loop programming" of the control to distribute the width of the pulses. The burden on the output filter is, thus, reduced. Sinusoidally distributing the ultrasonic carrier frequency pulses that are used to control the ultrasonic switching allows the reduction in the size of the output filter and improves closed loop response. These improvements are a result of the higher break point in the frequency response of the output filter and the higher sample data rate of the inverter. However, this approach still requires a large low frequency power transformer. Especially in the application of 50 to 60 Hz AC power, the size of the transformer completely dominated the size and weight of the power converter.